The objective of this research is to study the biochemical and physiological changes in selenium deficient subhuman primates, using 12 selenium depleted pregnant Rhesus monkeys. Four of these will be fed basal diet (with casein, soybean meal and torula yeast as the protein sources) with additions of 0.2 ppm selenium. The remaining eight monkeys will be fed the selenium deficient basal diets containing either casein, soybean meal plus torula yeast (30 ppb Se), or soybean meal plus torula yeast (15ppb), or just torula yeast (15 ppb) as the protein sources. Blood will be taken at monhly or biomonthly intervals from both the adult and young monkeys for selenium and vitamin E analyses, and for enzyme assays. A complete blood count, and measurements of erythrocyte glutathione peroxidase activity, plasma glutamic-oxalacetic transaminase, lactic dehydrogenase, creatine phosphokinase, and ornithine carbamyl transferase activities are planned on each blood sample. At specified intervals, the influence of selenium deficiency on blood cholesterol and lipoprotein levels, neutrophilic function, in vivo and in vitro immunity, excretion of fecal mutagen, sleep time, and electrocardiograms will be evaluated in both young and adult monkeys. At the end of this study, the young monkeys will be killed and a complete necropsy and histopathological study performed. Glutathione peroxidase activity and selenium content will be determined on their tissues. The microsomal hemoproteins levels and the activities of some of the enzymes in the mixed function oxidase system will also be determined in livers from these young monkeys.